1. Field of the Invention
The present invention relates to a watertight construction for ends of wires where a core convergent splice portion is formed. The invention also relates to a method for forming the watertight construction. The invention is particularly designed to prevent water from penetrating through clearances between wires.
2. Description of the Related Art
A prior art wiring harness for an automotive vehicle or the like includes wires with cores that are connected to form a splice portion for the branch connection of circuits. The prior art includes a method according to which cores at the ends of the wires are exposed, collected and connected to form an end convergent splice portion. This prior art method has been adopted widely in recent years because it has few operation steps and is efficient.
The end convergent splice portion needs to be watertight and insulated securely from the outside. A variety of proposals have been made to realize such watertightness and insulation.
For example, Japanese Unexamined Patent Publication No. 9-55278 discloses a method according to which cores are exposed at ends of a plurality of wires and then are connected to form a convergent splice portion. Ends of the wires in proximity to the splice are immersed in a cyanogen or cyanoacrylate adhesive of low viscosity (30 cp or lower) which is commercially available and which generally is called "instantaneous adhesive". This method step causes the adhesive to penetrate into clearances between the cores and between the wires. The ends of the plurality of wires then are covered by a protection tube made of a synthetic resin.
According to the above described prior art method, the adhesive penetrates into the clearances between the cores at the ends of the plurality of wires and solidifies while covering the respective wires. Thus portions of the cores that were exposed by removal of the insulation coatings are completely covered by the adhesive. Even if water should enter the protection tube made of the synthetic resin from the outside, the adhesive-covered cores are not likely to rust. Further, the low viscosity adhesive that penetrated into the clearances between the cores also enters the insulation coatings by capillary action and solidifies therein. Consequently, any moisture that has entered the protection tube does not enter the insulation coating, thereby achieving some degree of watertightness.
However, the watertight construction formed according to the above described prior art method could not realize reproducible and acceptably secure watertightness. Specifically, this prior art method assumes that, when the ends of the plurality of wires where the core convergent splice portion is formed are immersed in the cyanoacrylate adhesive, a relatively large amount of adhesive deposits on the outer surface of the insulation coatings of the wires. Furthermore, this prior art method assumes that the protection tube can be mounted easily on the wire ends in this state because the relatively large amount of adhesive acts as a lubricant. In reality, however, these assumptions are not accurate. In particular, with reference to FIG. 8, the prior art process includes immersing the wire ends in adhesive 11 of such a low viscosity that the adhesive 11 penetrates into the clearances between cores 1 by capillary action. If the wires w then are removed from the adhesive, the adhesive 11 runs down the outer surfaces of wires w to a convergent splice portion 1A of the cores 1 at the bottom end. The adhesive 11 then drips as shown in FIG. 8. A thin adhesive layer lla remains on the outer surfaces of the insulation coatings 2 of the wires w, and instantaneously solidifies upon efficiently absorbing moisture in the air because the adhesive has a low viscosity and the insulation coatings 2 of the wires normally are made of vinyl chloride which has a poor adhesiveness to the cyanoacrylate adhesive. On the other hand, relatively large clearances 13 are formed between the cores la in proximity to ends 2a of the insulation coatings 2. Relatively large clearances 14 also are formed between the insulation coatings 2, since the wires are loose in a position distanced from the convergent splice portion 1A of the wires w. As a result, adhesive pools 11b are formed in these relatively large clearances as shown in FIGS. 9(A) and 9(B). Cyanoacrylate adhesives (instantaneous adhesives) solidify upon absorbing a moisture. Accordingly, if an adhesive pool (droplet) is formed, it does not solidify, even if left for a long time, because the moisture is not absorbed by the inner part of the adhesive pool.
Thus, according to the prior art, the thin layer of the adhesive 11 solidifies on the outer surfaces of the insulation coatings 2 of the wires w before the protection tube is mounted. The protection tube then is mounted with the adhesive pools 11b formed between the cores 1 in proximity to the ends 2a of the insulation coatings 2 and between the insulation coatings 2. Accordingly, clearances s are formed between the thin layers of the adhesive 11a formed on the outer surface of the insulation coatings 2 of the wires w and the inner surface of the protection tube t as shown in FIG. 10. Additionally the unsolidified adhesive pools 11b exist between the cores 1 in proximity to the ends 2a of the insulation coating 2 and between the insulation coatings 2 of the wires w (see FIG. 9). Therefore, water or moisture may enter the protection tube t through the clearances s between the protection tube t and the wires w. The water or moisture may then enter the clearances 14 between the insulation coatings 2 of the wires w and the clearances 13 between the cores 1. As a result, the entrance of water or moisture cannot be prevented securely by the prior art method.
In view of the above, the prior art has taken measures to completely fill with solidified resin the clearances between the outer surfaces of the wires and the inner surface of the protection tube, as well as the clearances between the insulation coatings of the wires. As shown in FIG. 11, a plurality of wires w forming a core convergent splice portion 1A at their ends are separated at the opposite sides of a partition plate 20a of a wire retaining block 20. The prior art block 20 is manufactured by Kabushiki Kaisha Raychem and sold under the product name RayBlock. The block 20 is made of a thermoplastic resin and has an E-shaped cross section. The shape shown in FIG. 11 enables the block 20 to be mounted in specified positions on the insulation coatings 2. After the wire ends are immersed in an adhesive and pulled up therefrom, a heat-shrinkable tube 30 having a hot-melt layer 30a on its inner surface is mounted to cover the wire retaining block 20 (see FIG. 12). The block 20 and the hot-melt layer 30a then are melted by applying heat. The melted thermoplastic resin of the block 20 and the hot melt layer 30a fills the clearances between the outer surfaces of the wires w and the protection tube 30, as well as the clearances between the insulation coatings 2 of the wires w (see FIG. 13). The melted thermoplastic resin then is permitted to solidify in these clearances. According to this method, the melted block 20 penetrates into the clearances between the insulation coatings 2 of the wires w and solidifies there, whereas the melted hot-melt layer 30a penetrates into the clearances between the insulation coatings 2 of the outermost wires w and the inner surface of the protection tube 30 and solidifies there. In this way, the aforementioned problem of the prior art method disclosed in the Japanese Unexamined Patent Publication No. 9-55278 can be solved. However, the prior art method of FIGS. 11-13 has its own problems in that the wire retaining block 20 made of the thermoplastic resin is expensive and that work efficiency is poor because of extra operations of separating the wires in the wire retaining block and mounting the heat-shrinkable tube on the wires so as to cover the block.
The present invention was developed to solve the above problems, and an object thereof is to provide a watertight construction which can securely protect ends of wires where a core convergent splice portion is formed and can inexpensively be formed by a simple operation.